A display is an indispensable interface that allows human-machine communication for electronic products, such as mobile phones, personal digital assistants, tablets, and notebooks. Recently, touch panels have been integrated with displays in various electronic products; users can conduct various operations through touching the panel in a more convenient fashion. How to provide products that are lighter and thinner and have better visual effects has always been one of the main objectives behind the developments within the electronics industry, and the same is true for the new generation of products that incorporate laminated touch panel/display components.
Optical bonding is bonding of two or more optical components together using a clear optical refractive index-matched adhesive. In its simplest form, optical bonding eliminates the air gap between the a transparent cover, such as a cover glass, and the underlying display, such as an LCD, for example. Removing the air gap eliminates two reflective surfaces and reduces specular reflection. To prevent affecting visual effects, such adhesives should exhibit proper optical properties such as proper light transmittance and refractive index. When light travels through different mediums, reflection occurs due to the difference in refractive indexes, thereby affecting light's extraction efficiency. Optical devices, such as thin film and glass, usually have a relatively high refractive index. For example, the refractive index of glass is about 1.52. A disadvantage to many clear adhesives is that their refractive index is not sufficiently close to the glass substrates to which they may be applied. A common optically-transparent adhesive, such as acrylate adhesive, has a refractive index of about 1.4 to 1.45, which cannot meet current industry standards.
Fully hydrogenated C4 diol resins enable the formulation of optically clear adhesives which are generally used today in touchscreen applications, such as current generation smart phones. Optically clear adhesives allow for touch screens based on capacitive technology to eliminate a traditional multi-layer construction, thereby allowing manufacturers to create thinner, lighter devices. Polybutadiene diols are well-defined, pure materials that when selectively hydrogenated, produce very clear diols which meet the tight refractive index and clarity specifications required for these applications. Polybutadiene-based adhesives also demonstrate excellent moisture resistance and flexibility over a wide temperature range, effectively protecting the sensitive optics and other electronic components of the device. Polybutadiene-based adhesives however have high viscosities making the handling and application of this material difficult unless the adhesives are diluted, and dilution may negatively impact the refractive index of the adhesive. Thus, there is a need for new and improved low viscosity optically-transparent adhesives with a high refractive index.